In the end it is going to be her grandchild and you could call it "Shalto Fortescue Tinky-winks 3rd" and she would still think it was a lovely name.

Maybe next time she suggests a name and focus on it. Say, so you really think that "Mavis" is a really nice name, OK we will consider it, but aren't making any guarantees. That way at least she thinks that she is being consulted. And she will be focussing on that name rather than any other suggestions that she made.